This invention relates to electrophoretic displays and additives that inhibit undesired photo-thermally induced polymerization reactions in electrophoretic media to improve the performance of the media when used in a display. For example, the additives of the invention can improve the thermal and optical reliability of the electrophoretic displays.
Particle-based electrophoretic displays have been the subject of intense research and development for a number of years. In such displays, a plurality of charged particles (sometimes referred to as pigment particles) move through a fluid under the influence of an electric field. The electric field is typically provided by a conductive film or a transistor, such as a field-effect transistor. Electrophoretic displays have good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Such electrophoretic displays have slower switching speeds than LCD displays, however, and electrophoretic displays are typically too slow to display real-time video. Additionally, the electrophoretic displays can be sluggish at low temperatures because the viscosity of the fluid limits the movement of the electrophoretic particles. Despite these shortcomings, electrophoretic displays can be found in everyday products such as electronic books (e-readers), mobile phones and mobile phone covers, smart cards, signs, watches, shelf labels, and flash drives.
An electrophoretic image display (EPID) typically comprises a pair of spaced-apart plate-like electrodes. At least one of the electrode plates, typically on the viewing side, is transparent. An electrophoretic fluid composed of a dielectric solvent with charged pigment particles dispersed therein is enclosed between the two electrode plates. An electrophoretic fluid may have one type of charged pigment particles dispersed in a solvent or solvent mixture of a contrasting color. In this case, when a voltage difference is imposed between the two electrode plates, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate can be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate to the opposite plate, thereby reversing the color. Alternatively, an electrophoretic fluid may have two types of pigment particles of contrasting colors and carrying opposite charges and the two types of pigment particles are dispersed in a clear solvent or solvent mixture. In this case, when a voltage difference is imposed between the two electrode plates, the two types of pigment particles would move to opposite ends (top or bottom) in a display cell. Thus, one of the colors of the two types of the pigment particles would be seen at the viewing side of the display cell.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT), E Ink Corporation, E Ink California, LLC, and related companies describe various technologies used in encapsulated and microcell electrophoretic and other electro-optic media. In a microcell electrophoretic display, the charged pigment particles are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. The technologies described in these patents and applications include:
(a) Electrophoretic particles, fluids and fluid additives; see for example U.S. Pat. Nos. 7,002,728 and 7,679,814;
(b) Microcell structures, wall materials, and methods of forming microcells; see for example U.S. Pat. Nos. 7,072,095 and 9,279,906;
(d) Methods for filling and sealing microcells; see for example U.S. Pat. Nos. 6,545,797; 6,751,008; 6,788,449; 6,831,770; 6,833,943; 6,859,302; 6,867,898; 6,914,714; 6,972,893; 7,005,468; 7,046,228; 7,052,571; 7,144,942; 7,166,182; 7,374,634; 7,385,751; 7,408,696; 7,522,332; 7,557,981; 7,560,004; 7,564,614; 7,572,491; 7,616,374; 7,684,108; 7,715,087; 7,715,088; 8,179,589; 8,361,356; 8,520,292; 8,625,188; 8,830,561; 9,081,250; and 9,346,987; and U.S. Patent Applications Publication Nos. 2002/0188053; 2004/0120024; 2004/0219306; 2006/0132897; 2006/0164715; 2006/0238489; 2007/0035497; 2007/0036919; 2007/0243332; 2015/0098124; and 2016/0109780;
(e) Films and sub-assemblies containing electro-optic materials; see for example U.S. Pat. Nos. 6,982,178 and 7,839,564;
(f) Backplanes, adhesive layers and other auxiliary layers and methods used in displays; see for example U.S. Pat. Nos. 7,116,318 and 7,535,624;
(g) Color formation and color adjustment; see for example U.S. Pat. Nos. 7,075,502 and 7,839,564;
(h) Methods for driving displays; see for example U.S. Pat. Nos. 7,012,600 and 7,453,445;
(i) Applications of displays; see for example U.S. Pat. Nos. 7,312,784 and 8,009,348; and
(j) Non-electrophoretic displays, as described in U.S. Pat. No. 6,241,921 and U.S. Patent Applications Publication No. 2015/0277160; and applications of encapsulation and microcell technology other than displays; see for example U.S. Patent Application Publications Nos. 2015/0005720 and 2016/0012710.
Many commercial electrophoretic media essentially display only two colors, with a gradient between the black and white extremes, known as “grayscale.” Such electrophoretic media either use a single type of electrophoretic particle having a first color in a colored fluid having a second, different color (in which case, the first color is displayed when the particles lie adjacent the viewing surface of the display and the second color is displayed when the particles are spaced from the viewing surface), or first and second types of electrophoretic particles having differing first and second colors in an uncolored fluid. In the latter case, the first color is displayed when the first type of particles lie adjacent the viewing surface of the display and the second color is displayed when the second type of particles lie adjacent the viewing surface). Typically the two colors are black and white.
If a full color display is desired, a color filter array may be deposited over the viewing surface of the monochrome (black and white) display. Displays with color filter arrays rely on area sharing and color blending to create color stimuli. The available display area is shared between three or four primary colors such as red/green/blue (RGB) or red/green/blue/white (RGBW), and the filters can be arranged in one-dimensional (stripe) or two-dimensional (2×2) repeat patterns. Other choices of primary colors or more than three primaries are also known in the art. The three (in the case of RGB displays) or four (in the case of RGBW displays) sub-pixels are chosen small enough so that at the intended viewing distance they visually blend together to a single pixel with a uniform color stimulus (‘color blending’).
Although seemingly simple, electrophoretic media and electrophoretic devices display complex behaviors. For instance, it has been discovered that simple “on/off” voltage pulses are insufficient to achieve high-quality text in electronic readers. Rather, complicated “waveforms” are needed to drive the particles between states and to assure that the new displayed text does not retain a memory of the previous text, i.e., a “ghost.” See, for example, U.S. Patent Application No. 20150213765. Compounded with the complexities of the electric fields, the internal phase, i.e., the mixture of particles (pigment) and fluid, can exhibit unexpected behavior due to interactions between charged species and the surrounding environment (such as an encapsulation medium) upon the application of an electric field. Additionally, unexpected behaviors may result from impurities in the fluid, pigments, or encapsulation medium. Accordingly, it is difficult to predict how an electrophoretic display will respond to variations in the internal phase composition.
One particular disadvantage identified in present displays is unwanted reactions between the pigment particles, as well as the potential reactions between pigment particles and the polymeric material used to form the microcells. Pigment particles are commonly coated with polymers in order to impart certain functionality into the surface of the particles. When the polymeric particle coatings or microcell polymeric film contain (meth)acrylate groups, it is possible that the particles may cure together and agglomerate or that the polymeric material of the pigment coatings and microcells may crosslink when exposed to heat or UV light, for example, thus inhibiting/preventing the mobility of the pigment.
Thus, there is a need for improved electrophoretic media and displays in which the media is incorporated.